The present invention relates to transistorized differential transconductance amplifiers, and more particularly to differential f.sub.T doubler amplifiers which are compensated in some manner to reduce nonlinear error currents.
U.S. Pat. No. 3,633,120 a differential transconductance amplifier which is now commonly known as an "f.sub.T doubler amplifier" or simply as an "f.sub.T doubler". The basis for this name comes from the operation of the amplifier. At high frequencies, the gain of a simple differential pair transconductance amplifier exhibits gain rolloff at approximately 6 dB per octave. The frequency at which the gain becomes unity is designated f.sub.T or the unity gain frequency. Due to the configuration of the f.sub.T doubler circuit, the same amount of voltage input will provide twice the amount of current output at the unity gain frequency. Also, because of the nature of the gain rolloff, the f.sub.T doubler circuit will provide the same amount of current output for a given input voltage as the simple differential pair transconductance amplifier at twice the unity gain frequency.
Although the f.sub.T doubler increases the frequency response beyond that of the simple differential pair transconductance amplifier, it retains the disadvantages of imprecise gain linearity. U.S. Pat. No. 4,267,516 teaches a differential f.sub.T doubler amplifier circuit having an error correction amplifier. This circuit retains the enhanced frequency response of the f.sub.T doubler, but has improved linearity. One axiom of using error correction amplifiers is that the error correction amplifier should not introduce any of its own error. Therefore, significant bias current must be used to linearize the error correction amplifier. This bias current, however, is not reused in the main amplifier and substantially increases the overall power requirements of the amplifier.
What is desired is a transconductance amplifier which retains the enhanced frequency response of the f.sub.T doubler amplifier, but has improved linearity without significantly increasing power requirements.